Inkjet printers have printheads configured with a plurality of inkjets that eject liquid ink onto an image receiving member. The ink may be stored in reservoirs located within cartridges installed in the printer. Such ink may be aqueous, oil, solvent-based, or UV curable ink or an ink emulsion. Other inkjet printers receive ink in a solid form and then melt the solid ink to generate liquid ink for ejection onto the imaging member. In these solid ink printers, the solid ink may be in the form of pellets, ink sticks, granules, pastilles, or other shapes. The solid ink pellets or ink sticks are typically placed in an ink loader and delivered through a feed chute or channel to a melting device that melts the ink. The melted ink is then collected in a reservoir and supplied to one or more printheads through a conduit or the like. In other inkjet printers, ink may be supplied in a gel form. The gel is also heated to a predetermined temperature to alter the viscosity of the ink so the ink is suitable for ejection by a printhead.
A typical full width scan inkjet printer uses one or more printheads. Each printhead typically contains an array of individual nozzles for ejecting drops of ink across an open gap to an image receiving member to form an image. The image receiving member may be a continuous web of recording media, a series of media sheets, or the image receiving member may be a rotating surface, such as a print drum or endless belt. Images printed on a rotating surface are later transferred to recording media by mechanical force in a transfix nip formed by the rotating surface and a transfix roller. In an inkjet printhead, individual piezoelectric, thermal, or acoustic actuators generate mechanical forces that expel ink through an orifice from an ink filled conduit in response to an electrical voltage signal, sometimes called a firing signal. The amplitude, frequency, or duration of the signals affects the amount of ink ejected in each drop. The firing signal is generated by a printhead controller with reference to electronic image data. An inkjet printer forms an ink image on an image receiving surface with reference to the electronic image data by printing a pattern of individual ink drops at particular locations on the image receiving surface. The locations where the ink drops land are sometimes called “ink drop locations,” “ink drop positions,” or “pixels.” Thus, a printing operation can be viewed as the placement of ink drops on an image receiving surface with reference to electronic image data.
In order for the printed ink images to correspond closely to the image data, both in terms of fidelity to the image objects and the colors represented by the image data, the printheads must be registered with reference to the imaging surface and with the other printheads in the printer. Registration of printheads is a process in which the printheads are operated to eject ink in a known pattern and then the printed image of the ejected ink is analyzed to determine the orientation of the printhead with reference to the imaging surface and with reference to the other printheads in the printer. Operating the printheads in a printer to eject ink in correspondence with image data presumes that the printheads are level with a width across the image receiving member and that all of the inkjet ejectors in the printhead are operational. The presumptions regarding the orientations of the printheads, however, cannot be assumed, but must be verified. Additionally, if the conditions for proper operation of the printheads cannot be verified, the analysis of the printed image should generate data that can be used either to adjust the printheads so they better conform to the presumed conditions for printing or to compensate for the deviations of the printheads from the presumed conditions.
Analysis of printed images is performed with reference to two directions. “Process direction” refers to the direction in which the image receiving member is moving as the imaging surface passes the printhead to receive the ejected ink and “cross-process direction” refers to the direction across the width of the image receiving member that is perpendicular to the process direction. In order to analyze a printed image, a test pattern needs to be generated so determinations can be made as to whether the inkjets operated to eject ink did, in fact, eject ink and whether the ejected ink landed where the ink would have landed if the printhead was oriented correctly with reference to the image receiving member and the other printheads in the printer.
During a process direction registration operation, the inkjets in different printheads in the printer form predetermined patterns, which are referred to as “test patterns,” on the image receiving surface. Each inkjet ejects a plurality of drops in rapid succession as the image receiving surface moves in the process direction to form the test pattern with an arrangement of printed dashes, where each dash includes the ink drops ejected from a single inkjet and arranged in the process direction. An optical sensor in the printer generates image data corresponding to the printed dashes in the test pattern, and the printer adjusts the time of operation for inkjets in each of the printheads so that ink drops from multiple print heads are aligned in the process direction to enable production of high quality printed images.
Existing process direction registration techniques begin to lose effectiveness as the linear velocity of the image receiving surface increases. For example, in some printer embodiments existing process direction registration techniques become less effective as the linear velocity of a paper media web moving past the printheads in the process direction approaches and exceeds approximately 152 meters per minute (500 feet per minute). Increased image receiving surface speeds produce a corresponding increase in the throughput of the printer, but may also decrease the quality of printed images. For example, the increased media web velocity accentuates process direction drop placement errors because the media web moves a longer distance during a given time period. Thus, a time offset between inkjets in one or more printheads that is acceptable for use in lower-speed printer configurations is no longer acceptable as the linear velocity of the media web increases. Additionally, drop placement measurements extracted from the existing printed test patterns lose accuracy when the optical sensor in the printer generates image data of the test patterns at the increased web velocity due to decreased process direction resolution that results in aliasing of the printed dashes in the image data. Consequently, improved methods for performing process direction registration for printheads would be beneficial.